121
|
What should be voltage limit for adjacent
commutator segments at no load and full load?
|
|
The voltage
limit for adjacent commutator segments at no load is limited up to 15 V and
it is about 30 V at full load.
|
122
|
Which are the factors to be considered while
selecting number of armature slots?
|
|
Factors
to be considered for armature slots
- Flux pulsation
- Cooling of
armature conductors
- Mechanical
difficulties
- Commutation
|
123
|
Describe the effect of number of slots on the
cooling of armature conductor.
|
|
Number of armature slots
- If there are
large number of slots for a given armature diameter, number of conductors per
slot is less.
- Therefore the cooling condition is better for large number of
armature slots.
|
124
|
Describe the effect of number of slots on the
pulsating losses.
|
|
Number
of slots – Pulsating losses
- The air gap
flux due to number of slots give rise to eddy current losses in the pole
faces and produces magnetic noise.
- As the number
of slots increase, noise level of the machine and pole face losses are
reduced.
|
125
|
Why the current density of the armature conductor
should be selected high?
|
|
Current
density – Armature conductor
- The size of
conductor reduces for higher value of current density for a given current.
- This will
reduce cost of armature copper conductor.
- As the size of
conductor reduces, the slot area requires for the conductor also reduces.
- Therefore the shallow slots give better commutation condition.
|
126
|
Which are the factors to be considered while deciding
the slot size?
|
|
Factors
to be considered for slot size
- Reactance
voltage
- Mechanical
difficulties
- Eddy current
loss
- Flux pulsation
- Excessive flux
density
|
127
|
Describe the effect of slot opening on the flux
pulsation in the DC machine.
|
|
The wide open
slot produces higher flux pulsation than the narrow open slot. This will
cause iron losses and poor commutation condition therefore it is preferred
that the slot open width is not so wide.
|
128
|
Describe the importance of depth of slot.
|
|
Depth
of slot
- As the depth
of slot increases, the specific permeance becomes high. This will result in
high reactance voltage.
|
129
|
What should be limit for slot depth to slot width
in the DC machine?
|
|
Slot
depth to width
- As the
reactance voltage increases, the slot depth to slot width in the non inter
polar machine is limited to 4.
- The deeper
slots are allowed in the inter polar machines because inter pole produces
commutating field which compensate reactance voltage.
|
130
|
On which factors the heights of the pole depends
in DC machines?
|
|
The height of
the pole depends upon mmf required for the pole and yoke at full load.
|
131
|
Comment : The shunt field winding and series
field winding occupies same space in the compound machine.
|
|
The series
field winding occupies only 20% space whereas the shunt field winding
occupies 80% space in the compound machine.
|
132
|
Describe the different types of resistance during
commutation?
|
|
Resistance
during commutation
- Brush contact
resistance
- Resistance
between commutator segments
- Resistance of
risers
- Armature coil
resistance
|
133
|
Whether the statically or dynamically induced emf
during process of commutation?
|
|
Therefore are
following emfs produce during commutation.
Statically
induced emf
- This emf is
produced because of the winding coil has self inductance owing to slot
leakage and overhang leakage.
Dynamically
induced emf
- The effect of
armature reaction produces the field at brush axis. This emf is called as
dynamically induced emf.
|
134
|
Explain the term : Linear or Straight line
commutation
|
|
Linear
commutation
- If the brush
contact resistance is kept very high, the current in the coil follows
straight line commutation.
- It means that
the current reversal complete in the commutation time.
|
135
|
Explain the term : Delay time
|
|
Delay
time
- It is time
elapses between start of commutation between two adjacent armature coils.
|
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